One type of inertial navigation system (INS) employs an inertial measurement unit (IMU) that is floated in gas. Floating the IMU in gas creates a near frictionless environment to enable the IMU to move in all directions. By enabling motion in all directions, complete calibration utilizing earth gravity and earth rate is possible. By floating the IMU in gas, the need for gimbals and ball bearings is eliminated, thereby reducing the complexity, size, and cost of the inertial navigation system. Also, by eliminating gimbals, ball bearings, and other moving physical structures, there is typically no wear on the physical structures from contact between rotating surfaces, which improves the accuracy and durability of such an INS. Examples of such an INS are described in the '184 application.
In one exemplary implementation of an INS that uses a gas-supported IMU, the IMU is housed within a spherical sensor block. Typically, such a spherical sensor block is formed as two hemispheres. The two hemispheres are attached to one another using a main shaft that extends from one hemisphere and is connected to the other hemisphere. In order to balance the two hemispheres, the main shaft includes a three dimensional balance assembly comprising a center shaft with two or more eccentric weighted shafts encompassing the center shaft. These weighted shafts can be used to balance the overall assembly. One example of such a spherical sensor block is described in the '902 patent.
When joining the two hemispheres of such a spherical sensor block together, it is important that distortion of the sphere be kept below a minimum threshold limit. It is also important to keep slippage between the hemispheres during high G level environments below a minimum threshold limit. One example of where this may be a concern is during the launch of a vehicle in which the sensor block is deployed. For example, the relative angular position of internal instruments housed within the sensor block must be held to very small tolerances during G loading. This dictates very precise alignment to be maintained between the two hemispheres. Distortion or slippage of the hemispheres would cause the sensor block to be less spherical, which could result in instrument axis alignment error. Minimizing tolerance conflicts between the two portions of the sphere helps to reduce shifting during loads or thermal excursion of the assembly.
The joining together of two portions of a sphere with an axle, as described in the '902 patent, typically puts a load on the sphere, which may distort the sphere. In some applications, such distortion may be beyond acceptable limits. The axle passes through the center of the spherical assembly and may interfere with internal components in some applications.
Another possible approach to joining the two hemispheres is using a tongue-in-groove mechanical joint. However, such mechanical joints often have tolerance conflicts or require match machining of two parts, which does not allow for interchangeability with other parts. Generally, it is desirable to manufacture the two hemispheres independent of one another so one can be interchangeable with another having a different design, manufacturing date, or source.
Also, as noted above, the sensor block must be balanced properly to enable free rotation. Typically, the sensor block must be disassembled to balance the sensor block. Adjustment of final fine balance from the outside of the assembled sphere is desirable in order to obtain consistent results with minimal assembly/disassembly time.